The use of synthetic fibre-reinforced petroleum based plastics for aerospace and automotive applications is creating problems with depleting oil reserves and growing ecological damage. Currently, materials used in aircrafts for various applications include glass fibrecarbon-fibre reinforced epoxyphenolic composites.
An answer in solving these problems may be provided by natural fibre-reinforced composites or biopolymers based on renewable resources. The major attractions of these composites are that they are lightweight (leading to energy savings), environmentally-friendly, fully degradable and sustainable, that is, they are truly ‘green’. Other advantages of the use of such composites are that it contributes to the greening of aircraft components, the implementation of REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) and is in line with the European Union's Clean Sky Initiative. The use of natural fibres, however, poses problems because of their flammability and smoke generation.
One of the challenges in the aviation sector is to address flame, smoke and toxicity (FST) requirements. It is highly critical that panels for aviation applications should comply with FST standards, e.g. as per the United States of America's Federal Aviation Authority's (FAA) airworthiness criteria. Generally, the main flame retardant agents are applied in the matrix polymer.
U.S. Pat. No. 5,309,690 elaborates on the development of a composite panel comprising of sheets of natural fibre, such as corrugated cardboard impregnated with a thermosetting resin. The panel also contains a cellular core which is sandwiched between the sheets of natural fibre and contains inorganic insulating material and a material in granular form that releases water at elevated temperatures. A flame retardant agent such as diammonium phosphate may be included in the liquid composition used to impregnate the sheets with resin. It is to be noted that the panel does not include a fire protective coating and that the natural fibres were not treated with a flame retardant agent prior to impregnation with the resin. Glue was used to bond the core to the sheets.
WO 2007/20657 is based on the manufacture of a natural fibre thermoset composite of high tensile strength, compressive strength, high cross breaking point and high water absorption properties. In this disclosure, the composite is manufactured by impregnation of bamboo and jute fibre in a slurry of resin solution and additives, followed by compression moulding. In this document, it is mentioned that additives (possibly flame retardant agents) can be added in the resin.
IN 200300400 describes the manufacture of a moulded natural fibre thermoset fire proof composite sheet. The method involves dissolving modified resin with cross linking agents, forming a slurry by mixing filler and additives in a resin solution and impregnating jute cloth of any form in the slurry followed by compression moulding.
IN 200300729 describes the manufacture of a moulded natural fibre thermoset fire proof composite sheet. The method involves dissolving resin in methanol with cross linking agents, forming slurry by mixing filler and additives in a resin solution and impregnating jute cloth of any form in the slurry followed by compression moulding.
EP 1842957 describes providing a flame retardant fibre sheet with flame retardancy by coating a sheet with poly-ammonium phosphate having an average degree of polymerization in the range of between 10 and 40. The sheet may include synthetic and/or natural fibres. To mould the fibre sheet, the fibres in the sheet may be bonded with a synthetic resin binder. The resin may be a thermosetting resin. In the illustrative examples provided in the specification, a resin binder is applied and the polyammonium phosphate is either added with a resin binder or is applied after the application of a resin binder. The fibre sheet may be moulded into a panel shape or other shape, generally by hot-press moulding. A plural number of sheets can be laminated together upon moulding.
EP 1369464 describes a flame retardant agent which is a phosphate-containing compound which does not contain a halogen. The specification describes the treatment of polyester fibre woven fabric by immersion thereof in a solution containing the flame retardant agent followed by heat treatment at a prescribed temperature. The specification also describes the manufacture of articles with polymer materials that have been treated with the flame retardant material, the flame retardant material having been added to the polymer when it is molten.
US 2010/0324192 (corresponding published applications including CA 2667407 and EP 2089456) describes a process for the improvement of flameproofing fibre composite materials containing fibre materials embedded in a polymer, e.g. phenol resin. Pre-pregs are manufactured, preferably by known pre-preg or SMC-tooling methods, with the surface of the fibre-composite material being covered with a layer which includes a flame-proofing material, and in this regard the specification describes the use of aluminium hydroxide as a flame-proofing material. Instead or in addition, the fibre material may be treated with a flame-proofing material by soaking, spraying, coating or other methods before embedding the fibres in the polymer, and in this regard the specification describes the use of a flame-proofing material which is supplied under the trade name Flavacon GP (sic). It is believed that “Flavacon” should read “Flacavon”, and it is believed to be a phosphorus-based flame retardant agent. More particularly, it is believed that the active ingredient is an organic phosphorus and nitrogen containing compound. An artefact manufactured with natural fibres in accordance with this method was found to exhibit the following heat release values: Heat release (peak, 5 min): 47 kW/m2 and Heat release (2 min): 60 kW/m2.
US 2006/0189236 describes a panel having a three-dimensional artistic design on its surface and the manufacture thereof. The panel includes a first and a second outer layer which each comprise of fire retardant material or material which has been treated such that the material is fire retardant. The layers of fire retardant materials can comprise paper, fabric, foam, honeycomb or paper-backed adhesive. For example, one of the layers may comprise of paper or fabric and the other layer may comprise of foam, honeycomb or paper. The layers can be bonded by means of a welding machine such as an ultrasonic sound machine or attached by a thermoplastic, thermoset, thermobond or other fire resistant adhesive. The production of the fire-retardant layers is not described, the illustrative examples provided in the specification describing the use of various commercially available materials.
U.S. Pat. No. 7,232,605 describes composite structural members (e.g. panels or beams) which include polymers arranged in a two- or three-dimensional cellular skeletal structure and reinforced with fibres, which may be natural, and with nano-scale clay particles. The invention seeks to overcome the lower material stiffness of biocomposites by the use of cellular and sandwich structures. The polymers can be thermoset. It is stated that clay particles can double the tensile modulus and strength of numerous thermoset resins and, in addition, make the resin less permeable to liquids and gases, more flame retardant and tougher. The specification describes the manufacture of cellular beams and plates in which green hemp fibres or chopped flax fibres were impregnated with unsaturated polyester resin, with cells being formed with the use of removable rods. After the impregnation, curing was effected in an oven. Hybrid cellular sandwich panels are also described, which include skins cured integrally with a cellular core, the skins comprising a thermoset polymer, which may be nano-clay reinforced, and a natural or synthetic fibre mat.
US 2007/0238379 describes ballistic resistant composites and articles formed therefrom for use in airplanes and other vehicles. A central layer, preferably comprising of an aerospace-specification grade honeycomb material, is positioned between panels comprising of a plurality of non-woven fibrous layers, and then moulded into a structural member. Various high strength fibres are mentioned as being suitable for the panels including polyethylene fibres, aramid fibres, polybenzazole fibres, polyolefin fibres, polyvinyl alcohol fibres, polyamide fibres, polyethylene terephthalate fibres, polyethylene naphthalate fibres, polyacrylonitrile fibres, liquid crystal copolyester fibres, glass fibres, carbon fibres and rigid rod fibres. The fibrous layers are coated or impregnated with a polymeric composition and consolidated to form the panel. The polymeric composition is preferably a thermosetting plastics material. The panels may be attached to the honeycomb layer by means of an adhesive, with the panels preferably being independently moulded or consolidated prior to attachment to the honeycomb layer. Optionally, one or more layers of fire resistant material, such as fibre glass, aramid paper or a fibrous material impregnated with a fire resistant composition, may be attached to one or more surfaces of the panels to provide fire resistance. Alternatively, a fire resistant additive may be blended with the polymeric composition which is coated on the fibres. It is stated that the composites of the invention are particularly useful for the formation of structural members of airplanes or other vehicles, such as doors or bulkhead structures.
Phosphoric acid and its salts have been used for a long time as flame retardants for cellulosic fibres. Diammonium phosphate and ammonium phosphate, in particular, being the most widely used non-durable flame retardants for cellulosics (see for example Lyons J. W. Cellulose: Textiles in The Chemistry & Uses of Fire Retardants, pp 169-170, Wiley Interscience, New York, 1970 and Lewin M. and Sello S. B. Flameproofing of Cellulosics in Lewin M., Atlas S. M. and Pearce E. M. (Eds.), Flame-Retardant Polymeric Materials, pp 23-24, Plenum Press, New York, 1975).
Matko et al. applied diammonium phosphate to lignocellulosic fillers in an aqueous solution, followed by drying under an infrared lamp (see Matko Sz., Toldy A., Keszei S., Anna P., Bertalan, Gy. and Marosi Gy, Flame Retardancy of Biodegradable Polymers and Biocomposites, Polymer Degradation and Stability, 88, pp 138-145, 2005). The lignocellulosic materials (fillers) were wood flake, of 1.2 mm size, and corn shell, of 3-12 mm size. The polymer matrix was polyurethane.
It is to be appreciated that most current work on flame retardancy of natural fibre reinforced composites is concerned mainly with thermoplastic resins such as polypropylene.
Jang et al. produced paper-sludge/phenolic composites which contained flame retardants selected from phosphate/halogen, halogenated and inorganic flame retardants (see Jang J., Chung H., Kim M. and Sung H., The Effect of Flame Retardants on the Flammability and Mechanical Properties of Papersludge/Phenolic Composites, Polymer Testing, 19, pp 267-279, 2000). The inorganic flame retardants were mixed with the resin whereas the phosphate/halogen combinations were dissolved in a solvent before addition to the paper-sludge.
What is ideally required is a method of fabricating an artefact such as a panel which is environmentally-friendly and which has suitable characteristics for use in aircrafts i.e. lightness of weight, adequate strength and compliance with fire, smoke and toxicity requirements.
In artefacts comprising of resin-fibre compounds, the use of natural fibres, although advantageous from the view of being environmentally friendly, presents particular challenges when the artefacts require suitable FST characteristics for use in aircraft. In particular, the FAA Airworthiness maximum allowable values for OSU heat release (peak, 5 min), OSU heat release (2 min) and smoke density for decorated panels are 65 kW/m2, 65 kW·min/m2 and 200 Ds respectively, and the AIRBUS maximum allowable values for OSU heat release (peak, 5 min), OSU heat release (2 min) and smoke density for panels in an undecorated form are 35 kW/m2, 35 kW·min/m2 and 20 Ds, respectively.
Natural fibres are problematic in that they are particularly flammable and thus tend to require more flame retardant treatment than synthetic fibres. However, flame retardant agents tend to negatively affect the physical properties of the material. For good fibre-matrix adhesion when natural fibres are used, a resin of low viscosity is required to enable adequate penetration of the fibres. However, the addition of flame-retardant agents to the resin tends to increase the viscosity of the resin and can thus lead to poor fibre-matrix adhesion. This limits the amount of flame retardant agent that can be added to the resin in order to obtain adequate flame retardant properties. It is, moreover, difficult to treat natural fibres in an environmentally-friendly manner. Non-halogenated flame retardants, although advantageous for environmental considerations, tend to be less effective than halogenated flame retardants. The use of non-halogenated flame retardants on cellulosic materials generally increases smoke production.
Thus, the fabrication of an artefact with natural fibres which has suitable FST characteristics for use in aircraft is problematic, and there is a need for improvement on existing fabrication methods and their products.